Nanogripper device and method for detecting that a sample is gripped by nanogripper device

ABSTRACT

A fixed electrode and a movable electrode to be used to drive each arm are formed at a drive unit. As a voltage is applied between the fixed electrode and the movable electrode, the movable electrode is caused to move by coulomb force, thereby driving the arm  3  in a closing operation. By detecting a change occurring in the electrostatic capacity between the fixed electrode and the movable electrode at this time, a decision can be made as to whether or not the sample has been gripped.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nanogripper device used to handle anultrasmall machine and a method for detecting that a sample is grippedby a nanogripper device.

2. Description of the Related Art

The robust progress in micro-machining technologies achieved by adoptingsemiconductor processing technologies has invigorated interest inresearch and development of ultrasmall machines. Ultrasmall machines onthe order of microns are handled by using ultrasmall pincettes(hereafter referred to as nanogrippers) (see Japanese Laid Open PatentPublication No. H7-52072). Various types of actuators such aselectrostatic actuators, thermal actuators and piezoelectric actuatorshave been proposed to be used to open/close nanogrippers.

For instance, electrostatic actuators in the related art include anactuator that employs a comb-shaped electrode. This actuatoropens/closes the arms as the voltage applied to the electrode iscontrolled. In addition, in the device disclosed in the publicationquoted above, an actuator that engages in reciprocal movement is used toopen/close the arms incrementally in predetermined steps.

SUMMARY OF THE INVENTION

However, when the nanogripper described above is used to hold amicron-order sample, the judgment as to whether or not the arms of thenanogripper have gripped the sample needs to be made by visuallychecking through a microscope. Such visual judgment as to whether or notthe arms have gripped the sample is not always reliable and thus, thereis a risk of the process moving onto a transfer operation even when itis not certain that the sample has been firmly gripped. In addition, thearm closing operation may continue even after the sample has becomegripped, and in such a case, the sample is subjected to excessivestress. In particular, when handling a biological sample, the continuousclosing operation may result in too much deformation of the sample dueto the gripping force of the arms.

It would be desirable to provide a nanogripper device which includes apair of arms that is opened and closed freely, a drive mechanism thatdrives the pair of arms to open/close the arms and a hold detection unitthat detects that a sample has been gripped with the pair of arms.

It would be desirable to provide a method for detecting that a sample isgripped by a nanogripper device having a pair of arms that can be openedand closed freely and an electrostatic actuator that drives the pair ofarms so as to open or close the pair of arms, including steps fordetecting an electrostatic capacity at the electrostatic actuator anddetecting that a sample has been gripped by the pair of arms based upona change occurring in the detected electrostatic capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective schematically showing the structure adopted inthe nanogripper device in an embodiment;

FIG. 2 is a detailed enlargement of the grip portions;

FIG. 3 is a detailed plan view of the gripper main unit;

FIG. 4 shows the area over which an arm and a movable electrode areconnected with each other;

FIG. 5 presents sectional views taken along A11-A12, B11-B12, C11-C12and D11-D12 in FIG. 3;

FIG. 6 illustrates the operation of the nanogripper device;

FIG. 7 shows the qualitative relationship between the applied voltageand the distance between the arms;

FIGS. 8A to 8D each show the positional relationship between the sampleand the arms during the gripping operation;

FIG. 9 shows the relationship between the distance between arms and theelectrostatic capacity;

FIG. 10 shows the relationship between the applied voltage and theelectrostatic capacity;

FIG. 11 shows points of inflection;

FIGS. 12A to 12C illustrate a method that may be adopted to determinethe electrostatic capacity in correspondence to a time constant, withFIG. 12A showing the detection circuit, FIG. 12B showing the inputvoltage waveform Vin and FIG. 12C showing the observed voltage waveformVout;

FIG. 13 shows an oscillation circuit;

FIGS. 14A to 14D illustrate the procedure for manufacturing ananogripper device;

FIGS. 15A to 15C illustrate manufacturing steps in continuation fromFIGS. 14A to 14D;

FIGS. 16A to 16D show manufacturing steps in continuation from FIGS. 15Ato 15D;

FIG. 17 is a perspective of the silicon substrate in FIG. 14C;

FIG. 18 is a perspective of the substrate with the resist pattern withthe aluminum layer having been removed;

FIG. 19 is a perspective of the resist pattern;

FIG. 20 is a perspective showing the shapes of the resist and thealuminum layer in FIG. 16A;

FIG. 21 shows the rear surface side of the base layer in FIG. 16B;

FIG. 22 shows the front surface side of the base layer in FIG. 16C;

FIG. 23 shows the circuit unit having the detection circuit constitutedwith an oscillation circuit:

FIG. 24A shows the change occurring in the applied voltage, FIG. 24Bshows the change in the oscillation frequency and FIG. 24C shows thechange in the count value, all observed during the arm closingoperation;

FIG. 25 shows the structure of the arithmetic operation circuit engagedin operation during gripping force detection;

FIG. 26A shows the arms engaged in operation to grip the sample and FIG.26B shows the relationship between the voltage applied to the electrodesand the extent to which the arms are caused to move; and

FIG. 27 shows the dimensions of the comb portion of an electrode.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a perspective schematically showing the structure of ananogripper device 2 in an embodiment. The nanogripper device 2 in FIG.1 is formed on a semiconductor substrate by adopting a micro-machiningtechnology achieved through the semiconductor processing technology. Aguard 4 is disposed via a link portion 5 in order to protect arms 3 ofthe nanogripper device 2.

When the nanogripper device 2 is to be used in operation, the guard 4 issnapped off the nanogripper device 2 at the link portion 5. The arms 3are caused to open/close along the horizontal direction on the drawingsheet surface, as indicated by an arrow R11 by a drive unit 6. Amicron-order sample is gripped with grip portions 3 a formed at thefront ends of the arms 3.

As explained later, detection circuits and an arithmetic operationcircuit are formed in a circuit unit 9. As described later, thenanogripper device 2 is formed on a silicon substrate by adoptingsemiconductor processing technology.

FIG. 2 is a detailed enlargement of the grip portions 3 a. The gripportions 3 a are each formed by reducing the thickness of the front endportion of the arm 3 in stages. The width W1 and the thickness t1 of thegrip portions 3 a used to grip a micron-order sample are set so as toachieve dimensions substantially equal to those of the sample, with W1set to, for instance, 1 to 30 μm and t1 set to, for instance, 1 to 25μm. Since the gripping operation by the arms 3 is executed within thevisual field of the microscope, the length L1 of the grip portions 3 ais set to approximately 100 μm, slightly greater than the length of thesample to facilitate observation of the sample and provide easy accessto the sample. The grip portions 3 a are each formed by grinding off theupper surface side of each arm 3 in a stage. Since the arms 3 grip thesample placed on a flat stage under normal circumstances, the lowersurfaces of the arms 3 are flat.

FIG. 3 is a detailed plan view of the nanogripper device 2. Thenanogripper device 2 is mounted on a carrier mechanism such as an XYZstage prior to use. After the nanogripper device 2 is mounted at thecarrier mechanism, the guard 4 is disengaged from the nanogripper device2 by bending the link portion 5, thereby readying the nanogripper 2 foruse.

As shown in FIG. 3, the drive unit 6 formed on a base body 7 constitutesan electrostatic actuator and includes a fixed electrode 60 a and amovable electrode 61 a used to drive the left side arm 3 and a fixedelectrode 60 b and a movable electrode 61 b used to drive the right sidearm 3. The fixed electrodes 60 a and 60 b and the movable electrodes 61a and 61 b extend along the vertical direction in FIG. 3, with thesurfaces of the fixed electrode 60 a and the movable electrode 61 a andthe surfaces of the fixed electrode 60 b and the movable electrode 61 bfacing opposite each other each assume the shape of comb teeth. Themovable electrodes 61 a and 61 b are each elastically fixed onto thebase body 7 via a support unit 62.

As a voltage is applied between an electrode terminal 80 of the fixedelectrode 60 a and an electrode terminal 81 of the movable electrode 61a, a coulomb force causes the movable electrode 61 a to move to theright in FIG. 3. As a voltage is applied between an electrode terminal82 of the fixed electrode 60 b and an electrode terminal 83 of themovable electrode 61 b, the movable electrode 61 b is caused to move tothe left in FIG. 3.

The arms 3 are each elastically fixed onto the base body 7 by a supportunit 63. An electrode terminal 84 is connected to each arm 3 via thesupport unit 63. The left side arm 3 is linked to the left-side movableelectrode 61 a through a linking member 8 disposed at the bottom of thearm 3. Likewise, the right side arm 3 is linked to the right sidemovable electrode 61 b through a linking member 8.

FIG. 4 is an enlargement of the area over which the arm 3 is linked withthe movable electrode 61 a. The arm 3 is linked to the movable electrode61 a via the linking member 8. It is to be noted that an insulatinglayer 102 is formed between the arm 3 and the linking member 8 andbetween the movable electrode 61 a and the linking member 8. As themovable electrode 61 a is caused to move by the coulomb force to theright in the figure, the arm 3, too, moves to the right insynchronization with the movement of the movable electrode.

An identical structure is assumed with regard to the right side arm 3,the fixed electrode 60 b and the movable electrode 61 b in FIG. 3,except that the left/right orientation is reversed. Thus, as adifference is induced between the potential at the electrode 60 b andthe potential at the electrode 61 b by applying voltages to theelectrode terminals 82 and 83, the right side arm 3 moves to the left inthe figure. As a result, the left and right arms 3 close and the sampleis gripped with the grip portions 3 a.

As shown in FIG. 3, a groove-like through-hole 7 a is formed in the basebody 7 over the area where the arms 3 are disposed and a rectangularthrough-hole 7 b is formed at the base body 7 over the area where thedrive unit 6 is disposed. The arms 3 and the drive unit 6 respectivelybridge and are supported over the through-holes 7 a and 7 b.

FIG. 5 illustrates the shapes of various sections of the nanogripperdevice 2, in sectional views taken along A11-A12, B11-B12, C11-C12 andD11-D12 in FIG. 3. As shown in the sectional view taken along A11-A12,the through-hole 7 a is formed under the arms 3. The drive unit 6 isformed on the base body 7 via the insulating layer 102. Likewise, thearms 3 and the drive unit 6 (the movable electrodes 61 a and 61 b)linked via the linking members 8, too, are formed on the base body 7 viathe insulating layer 102. The sectional view taken along D11-D12 showsan area where the electrode terminals 80 to 84 in FIG. 3 are present.The electrode terminals 80 to 84, too, are formed on the base body 7 viathe insulating layer 102.

As described above, the nanogripper device 2 is formed on a substrateassuming a three-layer structure which includes two silicon layerssandwiching an insulating layer, e.g., an SOI (silicon on insulator)substrate. The arms 3, the drive unit 6 and the electrode terminals 80to 84 are all formed from a single silicon layer. It is to be noted thatthe method for manufacturing the nanogripper device 2 is to be detailedlater.

<<Operation >>

FIG. 6 is a block diagram of the internal structure adopted in thecircuit unit 9 that controls the drive unit 6. FIG. 6 only includes apartial illustration of the drive unit 6, i.e., part of the fixedelectrode 60 a and the movable electrode 61 a used to drive the leftsidearm 3. The electrodes 60 a and 61 a constitute an electrostaticactuator. By applying a voltage to the electrode terminals 80 and 81 inFIG. 3, the movable electrode 61 a is driven. As explained earlier, themovable electrode 61 a is elastically fixed to the base body 7 via thesupport unit 62.

The circuit unit 9 includes a detection circuit 91A, a detection circuit91B, an arithmetic operation circuit 92 and a storage circuit 93. Anexternal DC source 10A is connected to the electrodes 60 a and 61 a viathe detection circuit 91A, whereas a DC source 10S is connected to theelectrodes 60 b and 61 b via the detection circuit 91B.

As explained later, the detection circuit 91A detects the electrostaticcapacity between the electrodes 60 a and 61 a, and the detection circuit918 detects the electrostatic capacity between the electrodes 60 b and61 b. The arithmetic operation circuit 92 calculates the distancebetween the arms 3 and the level of the gripping force achieved with thearms 3 based upon the electrostatic capacities detected with theindividual detection circuits 91A and 91B. Data needed in the arithmeticoperations executed at the arithmetic operation circuit 92 are stored inthe storage circuit 93. In addition, the arithmetic operation circuit 92outputs control signals based upon which the voltages output from the DCsources 10A and 10B are controlled. This structure enables independentcontrol of the left side arm 3 and the right side arm 3.

In the embodiment, the portions of the fixed electrode 60 a and themovable electrode 61 a facing opposite each other are formed in theshape of comb teeth. Thus, the electrodes can be disposed close to eachother, and it is possible to assure ample range of movement whilemaintaining a narrow gap between the electrodes. This allows the drivevoltage to be lowered. In addition, since the upward force and thedownward force acting symmetrically along the vertical direction canceleach other out, only the horizontal force is applied to the movableelectrode 61 a. In contrast, if the fixed electrode 60 a and the movableelectrode 61 a are respectively a simple plane parallel plate, asufficient level of coulomb force can be obtained only when the gapbetween the electrodes is narrow, and the coulomb force becomes weakeras the range of movement increases and the gap becomes greater, in whichcase, a higher voltage is required. Comb teeth 600 formed at the fixedelectrode 60 a and comb teeth 610 formed at the movable electrode 61 aalternately project toward the opposite electrodes. A satisfactory levelof drive force can be obtained by setting the gap between the electrodes60 a and 61 a in the order of 1 to several μm.

In a specific example of the dimensions that may be set for the combteeth 600 and 610, the distance between the electrodes 60 a and 61 achanges within a range of 18 μm to 28 μm when the arm 3 opens/closesover a 10 μm width, the comb teeth 600 and 610 each have a width of 3μm, the gap between the comb teeth 600 and 610 ranging along thevertical direction in the figure is 3 μm and the length of the combteeth 600 and 610 is set to 15 μm. In principle, the arm 3 can be madeto open/close over a distance of up to several tens of μm.

For instance, if the positive side of the DC source 10A is connected tothe fixed electrode 60 a and the negative side of the DC source 10A isconnected to the movable electrode 61 a, the fixed electrode 60 a ischarged with positive electricity and the movable electrode 61 a ischarged with negative electricity. Then, an attracting force is inducedbetween the electrodes 60 a and 61 a due to the coulomb force caused bythe negative charge and the positive charge, and the movable electrode61 a moves to the right in the figure against the elastic force of thesupport unit 62. As a result, the left side arm 3 is driven to the rightin the figure. The operation of the right side arm 3 is identical to theoperation of the left side arm 3, except that the left/right orientationis reversed. Namely, as a voltage is applied to the electrodes 60 b and61 b, the right side arm 3 is driven to the left in the figure. The pairof arms 3 is thus closed.

FIG. 7 shows the qualitative relationship between the voltage Va appliedto the electrodes 60 a and 61 a and the distance Da between the arms 3.Do represents the distance between the arms 3 in an initial state inwhich the voltage Va is equal to 0. With the dimensions of the combtooth portions at the electrodes 60 a and 61 a set as shown in FIG. 27,the electrostatic capacity C_(comb)(x) can be expressed as in expression(1) below as a function of the range of movement x over which themovable electrode 61 a moves. It is to be noted that ε₀, w, l₀, l, g, b,Va and N in expression (1) respectively represent the vacuum dielectricconstant, the width of each comb tooth, the initial distance between thetips of the comb teeth and the wall surface of the opposite electrode,the length of the comb teeth, the gap between the individual comb teeth,the thickness of the comb teeth, the applied voltage and the number ofcomb teeth. $\begin{matrix}{{C_{comb}(X)} = {ɛ_{0}{{bN}\left( {\frac{w}{l_{0} - x} + \frac{l - l_{0} + x}{g}} \right)}}} & (1)\end{matrix}$

Since the energy stored between the electrodes 60 a and 61 a isexpressed in {C_(comb)(x)V²/2 (joules)}, the coulomb force F_(comb)(x)induced between the electrodes 60 a and 61 a can be expressed in (2)below. $\begin{matrix}\begin{matrix}{{C_{comb}(X)} = {ɛ_{0}{{bN}\left( {\frac{w}{l_{0} - x} + \frac{l - l_{0} + x}{g}} \right)}}} \\{= {ɛ_{0}{{bNV}^{2}\left( {\frac{1}{2g} + \frac{w}{2\left( {l_{0} - x} \right)^{2}}} \right)}}}\end{matrix} & (2)\end{matrix}$

Since the movable electrode 61 a and the arm 3 are elastically supportedvia the support units 62 and 63 respectively, the movable electrode 61 amoves to a position at which the coulomb force F_(comb)(x) expressed in(2) is in balance with the elastic force resulting from the deformationoccurring at the support units 62 and 63 in response to the applicationof the voltage Va. Namely, the distance Da between the arms 3 changes asindicated with the curve L1 in FIG. 7 in correspondence to the level ofthe applied voltage Va.

In order to grip the sample with the arms 3, the gripper main unit 2 ismoved by the carrier mechanism so as to position the sample Sa betweenthe arms 3 as shown in FIG. 8A. Subsequently, as the applied voltage Vais increased to V1 and then to V2, the distance between the arms 3decreases to D1 and then to D2, as shown in FIGS. 8B and 8Crespectively. Then, as the level of the applied voltage reaches V3 andthe distance between the arms is D3, the arms 3 come in contact with thesample Sa and the sample Sa becomes gripped by the arms.

(Sample Hold Detection)

The electrostatic capacity C_(comb)(x) at the electrodes 60 a and 61 ais expressed as in (1) above. If the range of movement x in expression(1) for the electrostatic capacity C_(comb)(x) is substituted with thedistance Da between the arms 3, the electrostatic capacity Ca changesroughly as indicated by the curve L2 in FIG. 9. As the distance isreduced to D1, to D2 and then to D3, the electrostatic capacity Caincreases to C1, to C2 and then to C3. Since the electrostatic capacityCa increases when the arms 3 are closed by raising the applied voltageVa, a relationship such as that indicated with the curve L3 in FIG. 10exists between the applied voltage Va and the electrostatic capacity Ca.Namely, as the voltage Va is applied, the distance Da between the arms 3and the electrostatic capacity Ca between the electrodes 60 a and 61 aare determined in correspondence to the level of the voltage Va.

When the level of the applied voltage reaches V3, the arms 3 contact thesample Sa as shown in FIG. 8D. Assuming that the sample Sa is a rigidsubstance, the arms 3 will not be allowed to move further along theclosing direction and thus the distance Da remains unchanged even if thelevel of the applied voltage Va is further raised after the arms 3 gripthe sample Sa. Thus, the electrostatic capacity Ca assumes a constantvalue C3 as indicated by the dotted line L4 in FIG. 11.

If, on the other hand, the sample is a deformable sample such as abiological sample, the contact pressure of the arms 3 causes deformationof the sample as the applied voltage Va increases beyond V3 and thegripping force further increases. Under such circumstances, theelectrostatic capacity increases slightly above C3, as indicated by thecurve L5 in FIG. 11. In either case, a detection can be executed toascertain whether or not the sample Sa has been gripped with the arms 3by detecting a point of inflection P at which the curve L3 changes tothe curve L4 or the curve L5.

(Methods for Detecting Electrostatic Capacity Ca)

Next, methods that may be adopted when detecting the electrostaticcapacity Ca at a detection unit 623 are explained. In the followingexplanation, a method through which the electrostatic capacity isdetected based upon a time constant and a method through which theelectrostatic capacity is detected based upon an oscillation frequencyare described. First, the method through which the electrostaticcapacity Ca is determined based upon a time constant is explained. FIG.12A shows a detection circuit, with C indicating a capacitor formed withthe electrodes 60 a and 61 a. A voltage Vin having a rectangularwaveform is applied from a source 20 to the circuit in which a resisterR1 is connected in series to the capacitor Cap. FIG. 12B shows thewaveform of the voltage Vin. A volt meter 21 is used to measure thedifference between the voltages at the two ends of the capacitor Cap.

Since the circuit shown in FIG. 12A constitutes an RC circuit, thevoltage value Vout measured with the volt meter 21 can be expressed asin (3) below. Namely, the voltage waveform such as that shown in FIG.12C is measured with the volt meter 21. Since the resistance value Ra isknown in advance, the electrostatic capacity Ca can be determined incorrespondence to Ca·Ra calculated based upon expression (3).Vout=Vin {1−exp(t/Ca·Ra)}  (3)

Next, a method for determining the electrostatic capacity Ca based uponan oscillation frequency is explained. FIG. 13 shows an oscillationcircuit achieved by connecting in parallel a coil 22 and a phaseinversion amplifier 23. The circuit in this example is structured sothat the electrostatic capacity of the electrostatic actuatorconstitutes a parameter of the oscillation circuit. For instance, acapacitor constituted with the electrodes 60 a and 61 a is connected atone of positions indicated with reference numeral C1, C2 or C3. If thecapacitor is connected at the position C2, the capacitor Cap isconnected between the input of the phase inversion amplifier 23 and theground, whereas the capacitor is connected between the output of thephase inversion amplifier 23 and the ground if the capacitor isconnected at the position C3.

The frequency of an AC signal output from the oscillation circuit, i.e.,the oscillation frequency Fosc, can be calculated as expressed in (4)below. Accordingly, by detecting the oscillation frequency Fosc, theelectrostatic capacity Ca can be calculated based upon expression (4)below. When opening/closing the arms by varying the voltage V, theelectrostatic capacity Ca is calculated by detecting the frequency Foscsequentially over predetermined time intervals, and the curve L3 in FIG.11 can be obtained. After the sample Sa is gripped at the positioncorresponding to a point P, the electrostatic capacity Ca which iscalculated subsequently changes as indicated by the curve L4 or thecurve L5. Namely, by calculating the electrostatic capacity Ca basedupon the frequency Fosc, the point of inflection P can be detected.Fosc(Hz)=½·π(L·Ca)^(1/2)  (4)(Detection Executed by Using Oscillation Circuits)

FIG. 23 shows the circuit unit 9 having the detection circuits 91A and91B each constituted with the oscillation circuit shown in FIG. 13.Since the AC impedance at the DC sources 10 takes on a value close to 0Ω, the fixed electrodes 60 a and 60 b and the movable electrodes 61 aand 61 b constituting the actuators are shorted, disabling the operationof the oscillation circuits. Accordingly, resistors R1 are disposed eachin series to the DC source 10A or 10B. Since the impedance at theactuators is determined in correspondence to the value assumed at theresistors R1, the oscillation circuits can be engaged in operation byselecting a value equal to or greater than 1 (MΩ) for R1.

If an external force is applied to the arms 3 and the fixed electrodes60 a and 60 b come in contact with the movable electrodes 61 a and 61 bin a structure that does not include the resistors R1, the DC sources10A and 10B will be shorted to result in the electrodes becoming fusedor welded. However, by controlling the electrical current with theresistors R1, fusing of the electrodes and the like can be prevented.

Between each oscillation circuit and the corresponding DC sources 10A or10B, a capacitor Cp for DC cutoff is disposed With the capacitors Cp, aflow of DC current from the DC sources 1A and 10B into the oscillationcircuits or a flow of DC current from the oscillation circuits into theDC sources 10A and 10B can be prevented. Thus, it is desirable to setthe electrostatic capacities of the capacitors Cp to a valueconsiderably larger than the electrostatic capacity of the drive unit 6.

As explained earlier, the electrostatic capacities between the fixedelectrode 60 a and the movable electrode 61 a and between the fixedelectrode 60 b and the movable electrode 61 b each constitute part ofthe circuit constant of the corresponding oscillation circuit. For thisreason, frequencies F1 and F2 of AC signals (oscillation frequencyoutputs) output from the individual detection circuits 91A and 91Bchange as the electrostatic capacities between the fixed electrode 60 aand the movable electrode 61 a and between the fixed electrode 60 b andthe movable electrode 61 b change. These AC signals are both input to acounter circuit 920 at the arithmetic operation circuit 92.

When closing the arms 3 by applying a voltage to the fixed electrodes 60a and 60 b and the movable electrodes 61 a and 61 b at the drive unit 6,the voltage is increased in steps as shown in FIG. 24A. A timing controlcircuit 921 outputs a trigger signal to the DC sources 10A and 10B overtime intervals Δt, and in response, the DC sources 10A and 10B each stepup the voltage by ΔV each time the trigger signal is received. V3indicates the voltage at which the sample becomes gripped.

As shown in FIG. 24A, the gaps between the fixed electrode 60 a and themovable electrode 61 a and between the fixed electrode 60 b and themovable electrode 61 b become smaller in steps and the correspondingelectrostatic capacities, too, increase in steps as the applied voltageincrease in steps. As a result, the frequencies F1 and F2 of the signalsoutput from the detection circuits 91A and 91B change in steps over thetime intervals Δt, as shown in FIG. 24B. The timing control circuit 921outputs a trigger signal to the counter circuit 920 so as to synchronizethe timing with which the counter circuit 920 is engaged in a countingoperation with the time interval Δt shown in FIG. 24B.

A count value Fa (see FIG. 24C) having been counted at the countercircuit 920 is first stored into the storage circuit 93, and acomparison decision-making circuit 922 compares the count value Fa witha count value Fb obtained through the counting operation executed inresponse to the next trigger. The comparison decision-making circuit 922determines the difference (Fb−Fa) between the count value Fb and thecount value Fa and makes a decision as to whether or not the difference(Fb−Fa) is smaller than a predetermined threshold value. If it isdetermined that the difference (Fb−Fa) is smaller than the thresholdvalue, data indicating that the sample has been gripped are output.Since the electrostatic capacities become constant once the movement ofthe arms 3 stops, the difference (Fb−Fa) should assume the value 0 inprinciple. Accordingly, even with factors such as drift taken intoconsideration, it is preferable that the threshold value assumes a valueclose to 0.

In the related art, the state of the hold on an ultrasmall sample by ananogripper has to be visually verified. However, the nanogripper devicein the embodiment makes it possible to verify with ease and accuracywhether or not the sample Sa has been gripped by detecting theelectrostatic capacity Ca at the drive unit 6 constituting theelectrostatic actuators. In addition, by setting the applied voltage toa predetermined value, as explained later, after verifying that thesample has been gripped, it is possible to ensure that no excessivegripping force is applied to the sample.

(Gripping Force Ga)

Next, a method that may be adopted to calculate the gripping force isexplained. In this example, the gripping force Ga imparted by the arms 3is determined based upon expression (2). The distance Da between thearms 3 can be expressed as Da=D0−2x with x representing the distanceover which each arm 3 move and D0 representing the distance between thearms 3 when no voltage is applied. Accordingly, expression (2) can bemodified to expression (5) below. $\begin{matrix}{F = {ɛ_{0}{{bNV}^{2}\left( {\frac{1}{2g} + \frac{w}{2\left( {l_{0} + \frac{D - {D0}}{2}} \right)^{2}}} \right)}}} & (5)\end{matrix}$

The coulomb force Fa is expressed as a function Fa(V, D) of the appliedvoltage Va and the distance Da. The relationship between the appliedvoltage Va and the distance Da, which is determined by the coulomb forceFa and the level of elastic forces at the support units 62 and 63, canbe qualitatively indicated as in FIG. 7. With D3 representing thedistance between the arms when the applied voltage is at V3, thecorresponding coulomb force Fa(V3, D3) is in balance with the level ofthe elastic force imparted at the support units 62 and 63. Namely, thelevel of the elastic force imparted when the distance between the armsis D3 is equal to Fa(V3, D3) and likewise, the levels of the elasticforce imparted when the distance is D1 and D2 are respectively equal toFa(V1, D1) and Fa(V2, D2). The gripping force Ga is expressed as in (6)below, and Ga=0 if the applied voltage is V3 in the state shown in FIG.BD.(gripping force Ga)=(coulomb force Fa)−(elastic force)  (6)(When Sample Sa is Not Deformable)

In the case of a non-deformable sample Sa, the distance is sustained atD3 even when the applied voltage is increased up to V4 in FIG. 7 andthus, the electrostatic capacity, too, remains unchanged from C3 (seeFIG. 11). The coulomb force induced in this situation is Fa (V4, D3) andthe corresponding elastic force is Fa(V3, D3). As expression (5)indicates, Fa(V4, D3)>Fa(V3, D3), and with Ga(V4, D3) representing thecorresponding gripping force, expression (6) is written as Ga(V4,D3)=Fa(V4, D3)−Fa(V3, D3).

Thus, the gripping force Ga (V, D3) when Va>V3 is true for the appliedvoltage Va can be expressed as in (7) below by using the elastic forceFa(V3, D3) imparted at the point of inflection P (distance D3). It is tobe noted that the distance D3 between the arms corresponding to Fa(V3,D3) can be determined based upon a correlation of the electrostaticcapacity C3 achieved when the point of inflection P in FIG. 11 isdetected and the correlation shown in FIG. 9, and V3 in Fa(V3, D3) canbe determined based upon a correlation of the electrostatic capacity C3and the curve in FIG. 10. These correlations are all stored in a storageunit 625 in advance.Ga(V, D 3)=Fa(V, D 3)−Fa(V 3, D 3)  (7)(When Sample Sa is Deformable)

As indicated by the curve L5 in FIG. 11, the point P5 corresponds to thevoltage V4 applied when the sample Sa gripped by the arms 3 becomesdeformed. The distance D5 between the arms corresponding to the point P5can be determined based upon the electrostatic capacity C5 detectedunder such circumstances and the correlation shown in FIG. 9. SinceC3<C5<C4 in FIG. 11 is true, D3<D5<D4 is also true. The level of theelastic force imparted at the support units 62 and 63 when the distanceis D5 is Fa(V5, D5), and the level of the coulomb force corresponding tothe distance D5 between the arms and the applied voltage V4 is Fa(V4,D5). Since V5<V4, Fa(V5, D5)<Fa(V4, D5) can be deduced from expression(5). The corresponding gripping force can be expressed as the differencebetween the coulomb force and the elastic force, i.e., Fa(V4, D5)−Fa(V5,D5).

Accordingly, the gripping force Ga(V, D5) on the deformable sample Sa isexpressed as in (8) below by using the elastic force Fa(V5, D5) at apoint P5 (distance D5). It is to be noted that V5 can be determinedbased upon the electrostatic capacity C5 and the correlation shown inFIG. 10.Ga(V, D 5)=Fa(V, D 5)−Fa(V 5, D 5)  (8)

The gripping force is actually determined through the followingprocedure, regardless of whether or not the sample Sa is deformable.First, the electrostatic capacity is detected and based upon thedetected electrostatic capacity and the relationships shown in FIGS. 9and 10, the distance Da between the arms and the voltage Va aredetermined, and then the elastic force is calculated based upon Da andVa having been determined in expression (5). In addition, the level ofthe coulomb force which is actually at work is calculated incorrespondence to the distance between the arms determined based uponthe actual applied voltage Va and the actual electrostatic capacity.Lastly, the gripping force is determined by calculating the differencebetween the coulomb force and the elastic force.

The gripping force imparted when the sample is not deformable iscalculated as in (7), since the electrostatic capacity is C3, theelastic force is Fa (V3, D3) and the coulomb force actually at work isFa(V, D3). The gripping force imparted when the sample is deformable iscalculated as in (B) since the electrostatic capacity is C5, the elasticforce is Fa(V5, D5) and the coulomb force actually at work is Fa(V, D5).

As described above, since the level of the gripping force can beascertained, the nanogripper in the embodiment is enabled to operatewith the gripping force with a level corresponding to the type of thesample Sa. This feature is particularly advantageous when the sample isa biological sample, since damage to the biological sample due to anexcessive gripping force can be prevented.

The gripping force is calculated in a circuit achieved by modifying thecircuit shown in FIG. 23 to that shown in FIG. 25. A counter 925 countsthe number of triggers output to the DC sources 10A and 10B. The countvalue is stored into a resistor 924. For instance, the arms 3 may gripthe sample Sa when, following the initial state in which the appliedvoltage is at 0, nine trigger outputs have been counted, as shown inFIG. 26A. Each time the trigger is output, the applied voltage isincreased by ΔV.

In the range over which a linear relationship exists between the voltageand the displacement of the drive unit 6, the extent to which the arms 3need to move before gripping the sample is calculated as 9Δd (see FIG.265) with Δd representing the range of movement corresponding to eachΔV. When such a linear relationship does not exist, the range ofmovement is calculated as the sum of the distances, i.e., Δd1+Δd2+ . . .Δd. If the applied voltage keeps increasing in response to triggersignals output after the sample is gripped, the increase in the voltageoccurring after the sample is gripped is calculated as (Ct−9) ΔV with Ctrepresenting the count value indicating the number of trigger signalshaving been counted since the start of the applied voltage increase. Thegripping force can be calculated as explained earlier based upon thevoltage increase (Ct−9) ΔV. Alternatively, a specific level of grippingforce may be set in advance from the outside and once the gripping forcereaches the preset level, the trigger output may be stopped so as toensure that the applied voltage does not keep increasing after thesample is gripped.

(Production Process Through Which Nanogripper Device 2 is Manufactured)

Next, an explanation is given on a manufacturing method that may beadopted when forming the nanogripper device 2 by using an SOI (siliconon insulator) substrate. It is to be noted that the followingexplanation focuses on the method for forming the arms 3 and the driveunit 6, and an explanation or an illustration of the method for formingthe circuit 9 is not provided. The circuit unit 9 may be formed throughthe semiconductor processing technology in the same silicon layer asthat used to form the arms 3 and the drive unit 6, or circuit elementshaving been separately formed may be disposed onto the base body 7. Asubstrate 100 used to manufacture the nanogripper device 2 is a siliconsubstrate achieved by sequentially laminating a base layer 101constituted of single crystal silicon with a (110) orientation, aninsulating layer 102 constituted of silicon oxide and a silicon layer103 constituted of single crystal silicon with the (110) orientation.

Instead of an SOI substrate, a substrate having a single crystal siliconlayer deposited on a glass substrate, an amorphous silicon substrate, asubstrate having an SOI layer formed on a polysilicon substrate or thelike may be used as the silicon substrate 100. Namely, the base layer101 at the silicon substrate may adopt a multilayer structure, as longas the uppermost layer is the silicon layer 103 with the (110)orientation and the insulating layer 102 is formed under the siliconlayer 103.

The individual layers at the silicon substrate 100 may be formed so thatthe silicon layer 103, the insulating layer 102 and the base layer 101respectively achieve a 251 m thickness, a 1 μm thickness and a 300 μmthickness, for instance. In addition, an area over which a gripper is tobe formed on the silicon substrate 100 assumes a rectangular shaperanging over several millimeters both longitudinally and laterally. Inthe step shown in FIG. 14A, an aluminum layer 104 is formed so as toachieve a thickness of approximately 50 nm at the surface of the siliconlayer 103 through sputtering or vacuum deposition.

Next, as shown in FIG. 14B, a resist 105 with an approximately 2 μmthickness is formed on the surface of the aluminum layer 104, and then,a resist pattern 105 a shown in FIG. 14 c is formed by exposing anddeveloping the resist 105 through photolithographic method. FIG. 17 is aperspective of the silicon substrate 100, at which the resist pattern105 a corresponding to the arms, the guard 4, the drive unit 6 and thelike is formed over the upper surface of the aluminum layer 104. It isto be noted that FIG. 14C is a sectional view taken along F11-F12 inFIG. 17.

Next, as shown in FIG. 14D, the aluminum layer 104 is etched with amixed acid solution by using the resist pattern 105 a as a mask untilthe silicon layer 103 is exposed. Subsequently, through ICP-RIE(inductively coupled plasma-reactive ion etching), the silicon layer 103is anisotropically etched along the vertical direction. This etchingprocess is executed until the insulating layer 102 becomes exposed, andafter the etching process, the resist pattern 105 a and the aluminumlayer 104 are removed by using a mixed solution containing sulfuric acidand hydrogen peroxide (see FIG. 15A).

FIG. 18 is a perspective of the substrate 100 following the removal ofthe resist pattern 105 a and the aluminum layer 104. Over the insulatinglayer 102, a three-dimensional structure is formed with the singlesilicon layer 103. The three-dimensional structure includes portions 103a to constitute the arms 3, a portion 103 b to constitute the driveunits 6, portions 103 c to constitute the electrode terminals 80 to 84and a portion 103 d to constitute the guard 4.

Next, a resist 106 is applied so as to cover the insulating layer 102and the silicon layer 103 (103 a to 103 d) having become exposed (seeFIG. 15B). The resist coating 106 should be applied over a thickness ofapproximately 10 μm. Subsequently, a mask pattern is transferred ontothe resist 106 and is developed through photolithography and, as aresult, a resist pattern 106 a with the resist 106 removed over arectangular area at the front end side of the arm constituting portions103 a is formed as shown in FIG. 19. Then, the front end portions of thearm constituting portions 103 a are processed to achieve a shape andsize matching those of the target sample to be gripped with the gripperthrough an ICP-RIE process or the regular RIE process executed by usingthe resist pattern 106 a as a mask.

Next, as shown in FIG. 15C, the front and rear sides of the substrate100 are reversed and an aluminum layer 107 is formed over the surface ofthe base layer 101 through sputtering or vacuum deposition. The aluminumlayer 107 is formed so as to achieve a thickness of approximately 50 nm.After forming a resist 108 with a thickness of approximately 2 μm overthe aluminum layer 107, a resist pattern is formed throughphotolithography, and the aluminum layer 107 is etched with a mixed acidsolution by using the resist 108 as a mask (see FIG. 16A).

FIG. 20 is a perspective showing the shapes of the resist 108 and thealuminum layer 107. FIG. 16A is a sectional view taken along G11-G12 inFIG. 20, with sections of the arm portions 103 a constituted with thesilicon layer 103 shown on the lower side (toward the front surface) ofthe insulating layer 102. As FIG. 20 indicates, the resist 108 remainsnot removed at a portion R1 corresponding to the guard 4 in FIG. 1, atportions R2 corresponding to the link portions 5, at a portion R3corresponding to the base body 7 and at portions R4 corresponding to thelinking members 8 in FIG. 5, whereas the portions corresponding to thethrough-holes 7 a and 7 b in FIG. 5 have been removed, exposing the baselayer 101.

Subsequently, by using the resist 108 and the aluminum layer 107 formedover the base layer 101 as a mask, the base layer 101 is etched throughICP-RIE. The base layer 101 is anisotropically etched along the verticaldirection. The base layer is etched until the insulating layer 102becomes exposed. Upon completing the etching process, the resists 108and 106 and the aluminum layer 107 are removed (see FIG. 16B) with amixed solution containing sulfuric acid and hydrogen peroxide.

FIG. 21 shows the rear surface side of the base layer 101 in FIG. 16B.At the base layer 101, the base body 7 having the through-holes 7 a and7 b, the guard 4, the link portions 5 and the linking members 8 havebeen formed through etching. As a plurality of nanogripper devices 2each having a guard 4 are formed on the substrate 100 under normalcircumstances, they are divided into individual nanogripper devices 2through this etching process. Next, the insulating layer 102 constitutedof silicon oxide, having been exposed over the base body, is etched byusing a buffer hydrogen fluoride solution. As a result, the insulatinglayer 102 is removed except for the insulating layer present over areaswhere it is sandwiched between the silicon layer 103 and the base layer101 (see FIG. 16C).

FIG. 22 is a perspective of the front surface side of the base layer 101and FIG. 16C is a sectional view taken along H11-H12 in FIG. 22. Theinsulating layer 102 is present between the electrode portions 103 andthe base portion 101. Subsequently, a conductive film 109 constituted ofaluminum or the like is formed through vacuum deposition or the likeover the exposed base layer 101 and also over the silicon layer 103constituting the individual components. The conductive film 109 shouldbe formed to achieve a thickness equal to or less than 500 nm. While theproduction of the nanogripper device 2 in FIG. 1 is thus completed,additional processing may be executed on the grip portions 3 a with amachining device such as an FIB.

The present invention is not limited to the embodiment described above.For instance, while the hold on the sample Sa is detected and thedimensions of the sample Sa are measured by detecting the electrostaticcapacity Ca at the drive mechanism 6 constituting electrostaticactuators in the embodiment described above, the hold detection and thegripping force measurement may be executed by applying the voltage froma DC source 621 to the conductive film 109 formed over the arms 3 anddetecting a change occurring in the electrostatic capacity at the arms3, instead. In such a case, the drive unit 6 does not need to constitutean electrostatic actuator and may instead be a drive mechanism achievedby using piezoelectric elements or thermal expansion elements.

In addition, the oscillation circuit in FIG. 13 may include a quartzvibrator instead of the coil 22.

The disclosure of the following priority application is hereinincorporated by reference:

-   Japanese Patent Application No. 2004-210566 filed Jul. 16, 2004

1. A nanogripper device comprising: a pair of arms that is opened andclosed freely; a drive mechanism that drives the pair of arms toopen/close the arms; and a hold detection unit that detects that asample has been gripped with the pair of arms.
 2. A nanogripper deviceaccording to claim 1, wherein: the drive mechanism is an electrostaticactuator; the nanogripper device further comprises an electrostaticcapacity detection unit that detects an electrostatic capacity of theelectrostatic actuator; and the hold detection unit detects that thesample has been gripped with the pair of arms based upon a changeoccurring in the electrostatic capacity detected by the electrostaticcapacity detection unit while closing the arms.
 3. A nanogripper deviceaccording to claim 2, wherein: the hold detection unit judges that thepair of arms has gripped the sample upon detecting a point of inflectionin the change occurring in the electrostatic capacity detected by theelectrostatic capacity detection unit while closing the arms.
 4. Ananogripper device according to claim 3, further comprising: a storageunit in which a first correlation between a distance between the pair ofarms and the electrostatic capacity at the electrostatic actuator and asecond correlation between a voltage applied to the electrostaticactuator and the electrostatic capacity are stored in advance in memory;and a gripping force calculation unit that calculates a gripping forceimparted by the pair of arms gripping the sample based upon the voltageapplied to the electrostatic actuator, the electrostatic capacitydetected by the electrostatic capacity detection unit, the firstcorrelation and the second correlation.
 5. A nanogripper deviceaccording to claim 2, wherein: the electrostatic actuator includes apair of comb-shaped electrode the electrostatic capacity of which isdetected.
 6. A nanogripper device according to claim 3, wherein: theelectrostatic actuator includes a pair of comb-shaped electrode theelectrostatic capacity of which is detected.
 7. A nanogripper deviceaccording to claim 4, wherein: the electrostatic actuator includes apair of comb-shaped electrode the electrostatic capacity of which isdetected.
 8. A nanogripper device according to claim 2, wherein: thepair of arms, the electrostatic actuator and the electrostatic capacitydetection unit are formed at a semiconductor substrate through asemiconductor silicon processing technology.
 9. A nanogripper deviceaccording to claim 5, wherein: the pair of arms, the electrostaticactuator and the electrostatic capacity detection unit are formed at asemiconductor substrate through a semiconductor silicon processingtechnology.
 10. A nanogripper device according to claim 8, wherein: thepair of arms and the electrostatic actuator are linked via an insulatinglayer.
 11. A nanogripper device according to claim 9, wherein: the pairof arms and the electrostatic actuator are linked via an insulatinglayer.
 12. A method for detecting that a sample is gripped by ananogripper device having a pair of arms that can be opened and closedfreely and an electrostatic actuator that drives the pair of arms so asto open or close the pair of arms, comprising: detecting anelectrostatic capacity at the electrostatic actuator; and detecting thata sample has been gripped by the pair of arms based upon a changeoccurring in the detected electrostatic capacity.
 13. A method fordetecting that a sample is gripped by a nanogripper device according toclaim 12, wherein: the electrostatic actuator includes a pair ofcomb-shaped electrode the electrostatic capacity of which is detected.14. A method for detecting that a sample is gripped by a nanogripperdevice according to claim 12, wherein: a sample is a biological sample.15. A method for detecting that a sample is gripped by a nanogripperdevice according to claim 13, wherein: a sample is a biological sample.